In a pressurized-water nuclear power station, the very numerous tubes in the tube bundles in the steam generators must be subjected to periodical operations of checking and maintenance such, for example, as welding operations, the putting in place of shut-off plugs for neutalizing defective tubes, or operations on the lining inside tubes. These maintenance operations must be carried out from the waterbox, i.e., in a highly radioactive field. It is therefore necessary to employ remote-controlled apparatus in order to limit human interventions to the simple putting in place of the material. In addition, the steam generators generally have vertical tubes, i.e., the tubeplate is then on the "ceiling" of the waterbox.
It may likewise be necessary to operate upon the inner walls of the waterbox, e.g., in order to check their surface state.
Apparatus capable of being moved by remote control underneath the tubeplate of a steam generator have already been produced.
For example, the French patents published under Nos. 2.309.314 and 2.457.741 describe such apparatus having two perpendicular arms each equipped at both ends with an expandable gripper which can engage in the orifices in the tubeplate. By the actions of carriages and jacks, it is possible to give independently to each arm (a) motions perpendicular to the plate in order to engage or disengage the grippers in the orifices, and (b) step-by-step motions parallel with the plate, thus enabling independent movement of the apparatus by "creeping" underneath the plate. While in motion, the apparatus always remains suspended by at least two grippers and by four grippers in the working position.
Such apparatus displays the disadvantage of a rather large general bulk. More importantly, when an apparatus is in a working position suspended from its four grippers it is possible to operate upon only a very limited number of orifices in the plate. In the version of French Pat. No. 2.309.314, designed simply for guides for check probes, only four tubes are accessible; in the version of French Pat. No. 2.457.741, which enables the use of more complicated tools such, for example, as welding heads, only one orifice in the plate is within the field of action of the tool which can move only in a dihedron left free by the supporting arms. This is rather a nuisance every time that the same operation must be carried out upon a series of orifices in the same zone of the plate, because for each change of orifice the tool carrier must then be moved, i.e., one must disengage one arm, move it by one pitch and engage it again in the plate, and repeat the three operations on the other arm, with the result that the times for movement, which are dead times, are longer than the effective operational times.
Furthermore, each motion includes phases during which the tool carrier remains suspended by only two grippers, this greatly restricts the weight of the tools which can be employed.
It will further be observed that these apparatuses enable working only underneath the tubeplate itself, whereas operations may also be necessary on the inner walls of the waterbox.
Finally, although, for example, in the apparatus described in French Pat. No. 2.457.741 it has been possible to provide a small turret in order to receive various tools on standby, this in practice is only possible for small tools because of the limited load which can be supported by two grippers. In practice it is frequently necessary to change the tools on the tool carrier, making it necessary to enter the highly radioactive waterbox more often.